Protect multi-line strings from indentation in views_var_export()

This patch prevents user-defined multi-line strings like PHP eval code in argument handlers and views headers / footers from being indented when a view definition is exported

For example, a string like this:

'default_argument_php' => '$my_var = "foo";
if ($my_var == "foo") {
  // do some stuff
}',

Currently gets indented like this upon export:

'default_argument_php' => '$my_var = "foo";
  if ($my_var == "foo") {
    // do some stuff
  }',

A further export would cause additional indentation:

'default_argument_php' => '$my_var = "foo";
    if ($my_var == "foo") {
      // do some stuff
    }',

And so forth.

Indents in user-defined strings alter the whitespace content intended by the author and furthermore each additional export of a multi-line string appears as a substantive change in version control systems when no real modifications were made to the string.